The present invention relates to a molded polypropylene. More particularly, it relates to a molded article formed of a molding material which contains a polypropylene resin having an excellent balance among various physical properties including impact resistance, transparency, gloss, molding-shrinkage, stiffness and whitening resistance.
Polypropylene resins are used in various fields due to the excellent characteristics and inexpensiveness. Propylene homopolymers generally have a high stiffness, but are inferior in impact resistance, particularly at low temperatures.
A propylene block copolymer called xe2x80x9cblock copolymerxe2x80x9d in which low temperature impact resistance of a homopolymer is improved is known. The copolymer is prepared by incorporating a propylene homopolymer component at first, and then an ethylene propylene random copolymer component. This block copolymer is widely used in industrial fields including automobiles and home appliances.
Such block copolymers are excellent in impact resistance, but are inferior in transparency and gloss and more apt to produce whitening by impact of falling-down or a blow as compared with a homopolymer. Further, the copolymers have a high mold shrinkage factor, as is the case with the homopolymer. In order to improve this, a method of filling inorganic fillers such as talc or reducing an ethylene content in the random copolymer segment may be employed. The former, however, brings about a weight increase and poor appearance of the molded article due to the addition of a large amount of inorganic substance. The latter reduces the stiffness as well as the impact resistance at low temperatures, though the transparency and gloss are improved.
Relating to these problems, Japanese Patent Kokoku No. Hei 7-30145 discloses a propylene block copolymer comprising a crystalline polypropylene segment and an ethylene propylene random copolymer segment, wherein the crystalline polypropylene content is 55-95% by weight; an intrinsic viscosity ratio of both the segments, [xcex7]EP/[xcex7]PP, is 0.5-2.0; and the ethylene propylene random copolymer segment has a glass transition temperature of xe2x88x9230xc2x0 C. or lower. Further, JP-A-6-93061 discloses a propylene block copolymer obtained by melt-kneading a block copolymer which is prepared by polymerizing firstly 60-80% by weight, based on the whole polymerized amount, of a polymer mainly composed of propylene, and then an ethylene propylene copolymer segment having an ethylene content of 20-50% by weight. In the block copolymer, the ethylene propylene copolymer segment has an intrinsic viscosity [xcex7]B of 2.0 dl/g or more and the intrinsic viscosity ratio of the both segments, [xcex7]B/[xcex7]A, is 1.8 or less.
However, such propylene block copolymers have the ethylene propylene copolymer segment contents and an intrinsic viscosity ratio which are controlled almost in the same ranges. The former is improved in mechanical properties such as impact resistance and stiffness, but is still unsatisfactory in transparency, gloss and appearance. On the other hand, the latter is improved in impact resistance at low temperatures, blocking resistance and appearance, but is still unsatisfactory in stiffness. Molding shrinkage factor is also unsatisfactory in both cases.
The present invention has been made in the light of these problems in the conventional techniques, aiming to provide molded polypropylenes which are well balanced in impact resistance, transparency, gloss, molding shrinkage factor and whitening resistance.
In order to solve the above problems, the present inventors have intensively studied and found that the problem can be solved by dispersing a propylene/xcex1-olefin copolymer in a crystalline polypropylene in a specific condition, thus achieving the present invention.
The molded polypropylene of the present invention is formed of a resin composition comprising 10-60% by weight of a crystalline polypropylene and 90-40% by weight of a propylene/xcex1-olefin copolymer dispersed in the crystalline polypropylene. The molded polypropylene is characterized in that the above copolymer are dispersed to form plural domains elongated in the direction in which the resin composition flows during molding, and that each domain is connected to one or more other domains at least at one site.